How to Calculate Patio Size: What Area Do You Need?

When we laid our first proper patio, we made the classic mistake. We measured the space available, thought about how big we wanted the patio to be, and then compromised slightly to save money. We went with 3m x 3m instead of 4m x 4m.

It was fine. It held our table and four chairs. But every time we had people round for a barbecue we were negotiating around each other, and anyone who pushed their chair back too far was practically in the flower bed. We lived with it for five years before relaying it properly at the size it should have been in the first place.

The cost difference between a 3m x 3m and a 4m x 4m patio - in slabs and sub-base - is maybe 30-40% more material. Not nothing, but far less painful than doing the whole thing twice. Here is how to size it right first time.

Start with what goes on it

The most reliable way to size a patio is to start with the furniture you intend to put on it, not with a notional number of square metres.

Common outdoor furniture footprints:

These are the furniture footprints. The patio around them needs additional clearance.

The clearance rule

Around any dining table, allow at least 1 metre of clearance on all sides where chairs will be pushed out and people will walk. This is the bare minimum. 1.2-1.5m feels much more relaxed.

Example: 4-person dining set (1.5m x 2.0m)

Adding 1m clearance on all sides: (1.5 + 2m) x (2.0 + 2m) = 3.5m x 4.0m = 14m2

That is the minimum area required just for comfortable dining. Add space for the BBQ, any planters or pots, and a path to keep clear, and you are quickly at 18-20m2 before you have stretched at all.

Patio sizes by intended use

The "go one size up" principle

This is probably the single most consistent piece of advice from people who have built a patio: when torn between two sizes, go bigger.

Here is why it makes sense financially. The cost of a patio project is made up of:

• Excavation and labour (roughly the same regardless of size)
• Sub-base material (scales with area, but delivery cost is fixed)
• Slabs (direct cost per m2)

Going from 4m x 4m (16m2) to 5m x 4m (20m2) is 25% more area but the delivery, sub-base work and labour are similar. The extra slabs are the main addition. And in everyday use, you will notice the extra space constantly. You will not notice the slightly higher material cost after year one.

Going from 4m x 4m to 4m x 3m to save money is false economy if it means the patio is too cramped every time you use it.

Shape: rectangle is almost always best

Rectangular patios are the easiest to calculate, the most efficient use of slabs (less cutting), and usually give the best usable area for the footprint. Complex shapes - L-shapes, curves, unusual angles - cost more in cutting waste, take longer to lay and can look fussy rather than impressive.

If you want to define different zones (dining area and lounge area, for example), two connected rectangles at right angles are more practical than a complex single shape.

The main exception: if you have a fixed garden shape that a rectangle does not fit, a more complex outline can work well. Just add 15% wastage to your slab calculation instead of 10%.

Aspect ratio matters too

A very elongated patio (say, 8m x 2m) gives the same area as a 4m x 4m but feels completely different to use. A near-square or gently rectangular shape (ratio no more than roughly 2:1) works best for most outdoor dining setups.

Also consider the house orientation. If your patio is south or west facing, it will be in sun for more of the day. If it faces north, some extra area helps since you will never have the feeling of a warm, generous space that a smaller south-facing patio might have.

Now work out your slabs and sub-base

Once you have your dimensions, you have two main quantities to calculate:

Slabs: How many you need, which slab size, how many packs. Use the Patio Slab Calculator - enter your patio dimensions, slab size, joint gap and wastage to get a precise slab count and pack number.

Sub-base: Typically 75-100mm of compacted MOT Type 1 plus a mortar or sharp sand bed. Use the Sub Base Calculator or MOT Type 1 Calculator to plan that quantity alongside the slabs.

My tips on getting patio size right

A few things I wish someone had told me before our first attempt:

Lay it out with canes and string before committing. Measuring on paper is one thing. Actually standing in the garden with a rectangle of string marking the planned patio area gives you a completely different sense of the space. Do this with your furniture in the garden if possible. We added half a metre to one side of our second patio simply because the string marked out the original plan and it looked obviously tight.

Leave a path to keep clear. If the patio is against the house, you need to be able to walk around it - both for access and to comply with building regulations regarding DPC (damp-proof course) levels. Allow at least 600-900mm of clear path between any house wall and the nearest edge of the patio.

Think about winter as well as summer. A patio that faces west and gets afternoon sun is wonderful in summer. In November it looks different. Consider whether you want outdoor lighting, a sheltered corner, or some other winter-use feature, and factor that into the size and position.

The sub-base usually costs more than the slabs. This surprises a lot of people. Excavation, disposal, plate compactor hire, and 5-10 tonnes of MOT Type 1 can easily cost more than the slabs themselves. When you are choosing between a slightly bigger or slightly smaller patio, the slabs are a smaller proportion of the total cost than you might think.

Use the Patio Slab Calculator to get your slab counts once you have settled on a size.